1. Field of the Invention
This invention relates to a method for allocating channels when effecting connection in a cordless telephone.
2. Description of the Related Art
In the United States, 10 communication channels for communication are authorized for use in the cordless telephone by the FCC (the Federal Communications Commission). However, as the penetration into general usage of the cordless telephone increases, a shortage of communication channels has become a problem. In view of this, another 15 channels are scheduled to be added to the 10 channels mentioned above, which means 25 channels in total are due for use in the near future.
Every one of these channels is duplex, with up and down channels paired with each other.
As in the case of the extremely-low-power system adopted in Japan, the cordless telephone system in the United States is provided with no control channels; it only has communication channels, with the result that problems can be generated at the time of calling out from the handset.
Suppose, for example, a base unit B1 and a handset H1, which is associated therewith, are connected to each other by using a third channel, as shown in FIG. 3.
Further, suppose, at this time, the reachable area of the down-channel radio wave transmitted from the base unit B1 is AB1, which, as indicated by the solid line, is a substantially circular area, with the base unit B1 at the center thereof. The handset H1 is situated within this area AB1. Further, suppose the reachable area of the up-channel radio wave transmitted from the handset H1 is AH1. This area AH1, indicated by the broken line, is a substantially circular area, with the handset H1 at the center thereof. The base unit B1 is situated within this area AH1.
Further, suppose another base unit B2 is situated outside the reachable area AB1 of the radio wave of the base unit B1 and within the reachable area AH1 of the radio wave of the handset H1. Suppose, further, a handset H2, associated with the base unit B2, is situated outside the areas AB1 and AH1. Actually, however, the base units B1 and B2 are fixed stations, which means they are situated from the beginning at the positions shown in the drawing, with the handsets H1 and H2 having moved to their positions shown.
In this condition, suppose the handset H2 has sent a call-out request (request for connection) to the base unit B2 through the third up-channel. When seen from the handset H2, the third third up-channel paired with the down-channel is an available channel.
However, when seen from the base unit E-2, the third up-channel is busy, so that it cannot be used for communication according to the rules laid down by the FCC. Thus, the base unit B2 cannot open an up-channel for communication with the handset H2.
Similarly, when the base unit sends a call-in request to the handset, the down-channel may be busy when seen from the handset although the up-channel is available when seen from the base unit. In this case also, it is impossible, under the circumstances, to open a communication channel.